Field of the Invention
The present invention concerns a method for imaging a partial region of an examination subject in a magnetic resonance system, and a magnetic resonance system designed to implement such a method.
Description of the Prior Art
In a magnetic resonance system, the volume in which magnetic resonance data are acquired is limited in all three spatial directions due to physical and technical conditions (for example a limited magnetic field homogeneity and the nonlinearity of the gradient field). The acquisition volume—known as a “field of view” (FoV)—is therefore limited to a volume in which the aforementioned physical features lie within a predetermined tolerance range, and thus in which an image of the subject to be examined that is true to scale is possible with typical measurement sequences. The FoV is limited in the x-direction and y-direction, i.e. perpendicular to a longitudinal axis of a tunnel of the magnetic resonance system, but less significantly than the volume limited by the annular tunnel of the magnetic resonance system. In typical magnetic resonance systems a diameter of the annular tunnel amounts to approximately 60 cm, in contrast to which the diameter of the FoV that is typically used (in which the aforementioned physical features lie within the tolerance region) amounts to approximately 50 cm.
In many applications of magnetic resonance systems, this inadequacy (that no imaging of the measurement subject that is true to scale is possible in the boundary region of the tunnel of the magnetic resonance system) does not represent any great problem since, in basic magnetic resonance exposures, the region of the subject to be examined typically can be arranged in the magnetic resonance system such that this region is not located at the edge of the tunnel, but is optimally in the center of the tunnel (in what is known as the isocenter of the magnetic resonance system). Particularly in hybrid systems (for example a hybrid system having a magnetic resonance tomographic scanner and a positron emission tomographic scanner, known as an MR-PET hybrid system), it is frequently of great importance to also determine structures of the examination subject with as optimal a precision as possible in the boundary region. For example, the human attenuation correction is of decisive importance in an MR-PET hybrid system. The intensity attenuation of the photons emitted after an interaction of positrons and electrons on their way through absorbing tissue to the detector is determined with the human attenuation correction, and the received signal of the PET is corrected with precisely this attenuation. For this purpose, a magnetic resonance exposure is acquired that depicts the complete anatomy of the subject to be examined in the direction of the high-energy photons emitted by the positron emission scanner. It is also desired to acquire anatomy of the subject to be examined as precisely as possible in the boundary region of the tunnel of the hybrid system. When the subject to be examined is a patient, structures that are located in these regions are primarily the arms, which can be arranged near the inner tunnel wall of the hybrid system, in the boundary region.
In other applications of magnetic resonance systems—for example simply an examination of a particularly large (for example adipose) patient or stereotactic biopsies or other procedures to be executed during image monitoring—it can also be desirable to be able to expand the field of view to the boundary regions of the tunnel of the magnetic resonance system.
A method to determine an attitude of a partial region of an examination subject in a magnetic resonance system is provided in German Patent Application DE 10 2010 006 431.9. The partial region of the examination subject is arranged at the edge of the field of view of the magnetic resonance system. In this method, at least one slice position for a magnetic resonance image is determined automatically for which the B0 field at the edge of the magnetic resonance image satisfies a predetermined homogeneity criterion. Furthermore, a magnetic resonance image is acquired in this determined slice position that includes the partial region at the edge of the field of view. The attitude of the partial region of the examination subject is determined by the attitude of the partial region in the acquired magnetic resonance image.
Furthermore, in an article by Delso et al., a method has been proposed in order to compensate for the missing information in the magnetic resonance image (which information is missing due to the limitation of the field of view) by segmentation of the body contours using uncorrected PET data (G. Delso et al., Impact of limited MR field-of-view in simultaneous PET/MR acquisition, J. Nucl. Med. Meeting Abstracts, 2008; 49, 162P).
Since the field of view of a magnetic resonance system is limited to a volume in which the magnetic field inhomogeneity and the nonlinearity of the gradient field lie within specified regions, in the prior art various correction algorithms have been proposed in order to extend the field of view. For example, a gradient distortion correction is proposed in Langlois S. et al., MRI Geometric Distortion: a simple approach to correcting the effects of non-linear gradient fields, J. Magn. Reson. Imaging 1999, 9(6), 821-31, and in Doran S J et al., A complete distortion correction for MR images: I. Gradient warp correction, Phys. Med. Biol. 2005 Apr. 7, 50(7), 1343-61. Furthermore, a corresponding B0 field correction is proposed in Reinsberg S A, et al., A complete distortion correction for MR images: II. Rectification of static-field inhomogeneities by similarity-based profile mapping, Phys. Med. Biol., 2005 Jun. 7, 50(11), 2651-61.